reliability centered maintenance (rcm) for asset
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RELIABILITY CENTERED MAINTENANCE (RCM) FOR
ASSET MANAGEMENT IN ELECTRIC POWER
ANTHONY UWAKHONYE ADOGHE
A THESIS SUBMITTED IN THE DEPARTMENT OF
ELECTRICAL AND INFORMATION ENGINEERING TO THE
SCHOOL OF POSTGRADUATE STUDIES
IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR
THE AWARD OF DOCTOR OF PHILOSOPHY OF COVENANT
UNIVERSITY OTA, OGUN STATE, NIGERIA.
I hereby declare that I carried out the work reported in this thesis in
the Department of Electrical and Information Engineering, School of
Engineering and Technology, College of Science and Technology,
Covenant University, Ota, Nigeria under the supervision of Prof.
C.O.A. Awosope and Prof. J.C. Ekeh.
I also solemnly declare that no part of this report has been submitted
here or elsewhere in a previous application for award of a degree. All
sources of knowledge used have been duly acknowledged.
Engr. ADOGHE UWAKHONYE ANTHONY
This is to certify that this thesis is an original research work undertaken by
Anthony Uwakhonye ADOGHE (CU05GP0125) and approved by:
1. Name: Prof. C.O.A. Awosope
Signature: Date: 15th
2. Name: Prof. J.C. Ekeh
Signature: Date: 15th
3. Name: Prof. J.C. Ekeh
Head of Department
Signature: . Date: 15th
4. Name: Dr. T.O. Akinbulire
Signature: .. Date: 15th
This thesis is dedicated to God Almighty for his faithfulness and love towards
me and to the service of Humanity.
I am grateful to almighty God, the Author and finisher of my faith, for granting
me access to his ceaseless revelation, wisdom and favour that saw me through
my doctoral studies.
My sincere appreciation goes to the Chancellor, Dr. David Oyedepo for the
vision and mission of the University.
Also, special thanks to the Vice Chancelor, the Registrar, the Deans of Colleges,
the Heads of Department for their commitment and drive for excellence and
sound academic scholarship.
I also heartily appreciate and sincerely thank my supervisor, Prof. C.O.A.
Awosope, whose encouragement, guidance and support enabled the successful
completion of this thesis.
I owe my deepest gratitude to my co-supervisor who is also my Head of
Department Prof. J.C. Ekeh, for his good counsel, ever-ready willingness to
assist and motivate, and more importantly his critical review of the work and
useful suggestion in ensuring the success and speedy completion of this work.
It is my pleasure to thank Prof. James Katende, the Dean of the College of
Science and Technology (CST), for his support and encouragement all through
the course of this work. Special thanks to the Dean of post Graduate School,
Professor C.O. Awonuga and all my teachers at the postgraduate school. I thank
all my friends and senior colleagues in the Department of Electrical and
Information Engineering for their support and willingness to assist at very short
notice during the course of this work. Engr. Gab I. Ezeokoli the Abule-Egba
Business unit manager, Power Holding Company of Nigeria Plc is also highly
appreciated for given me open access to their maintenance data used for this
thesis. I also sincerely appreciate Dr.S.A. Daramola whose thesis provided useful
guide for my writing.
Lastly, but deliberately Iwant to specialy appreciate my wife and my children for
their understanding, support and contributions to the success of this endeavour,
God keep you all for me.
Title page i
Declaration . ii
Table of Contents vi
Lists of Symbols and Abbreviations x
List of Figures and Diagrams ........ xii
List of Tables xv
Chapter one: Introduction
1.1 Background information.....................................................................1
1.2 Research problem definition (statement of the problem)...................4
1.3 Aim and objectives of the study........................................................ 8
1.4 Research methodology...................................................................... 8
1.5 Significance of the study...................................................................9
1.6 Motivation of the study.....................................................................9
1.7 Expected contribution to knowledge ................................................11
1.8 Scope and limitation...........................................................................11
1.9 Thesis organization............................................................................12
Chapter two: Literature Review
2.1 Introduction..................................................................................... 14
2.2 Maintenance approaches............................................................... 16
2.3 The emergence of RCM............................................................... 17
2.4 Evolution of maintenance............................................................. 18
2.5 What is reliability? ................................................................. 26
2.6 Reliability centred maintenance.............................................. 28
2.7 Reliability engineering............................................................. 34
2.8 Reliability engineering process................................................ 35
2.9 Reliability evolution................................................................ 36
2.10 Limitation of RCM.................................................................. 39
2.11 Observations and Findings from the Literature Survey ..... 42
2.12 The proposal of reliability centred maintenance (RCM) for asset
management in electric power distribution system............................. 42
2.13 Summary ..................................................................................... 45
Chapter three: Theory of Reliability Evaluation
3.1 Introduction.................................................................................... 46
3.2 Definition and terminology .......................................................... 46
3.3 Applied reliability indices............................................................ 52
3.4 Maintenance Strategies.................................................................. 55
3.5 Choosing an appropriate distribution model ................................. 55
3.6 Modelling of life distribution function.......................................... 57
3.7 Exponentially distributed random variable................................... 59
3.8 Weibull-distributed random variable......................................... 59
3.9 Failure rate modelling for the RCM studies................................. 60
3.10 Method of reliability evaluation ............ 60
3.11 Ways for constructing the developed model.......................... 65
Chapter four: Application of RCM model to PHCN network
4.1 Introduction................................................................................. 80
4.2 The network topology description............................................... 81
4.3 Data collection and processing.................................................... 87
4.4 Modelling of failure and repair processes.................................... 100
Chapter five: Transformer Inspection and Maintenance:
5.1 Introduction.................................................................................... 104
5.2 Causes of transformer Failure............................................. 105
5.3 Transformer maintenance model....................................... 108
5.4 Equivalent mathematical models for transformer maintenance..112
5.5 Sensitivity analysis of inspection rate on mean time to
first failure (MTTFF). 115
5.6 Analysis of the mean time to first failure .. 120
Chapter six: Estimating the remaining life of the identified
6.1 Introduction................................................................................... 122
6.2 Assets life cycle...................................................................... 123
6.3 Techniques for asset management of transformers............ 124
6.4 Performing maintenance plans............................................... 129
6.5 Determination of the transition parameters for predicting the remaining life
of an asset (distribution transformer)........................................... 131
6.6 Determination of the steady - state probabilities.................... 133
6.7 Determination of the mean time to first failure...................... 135
6.8 Sensitivity analysis of failure rate on estimated remaining life of distribution
6.9 Discussion and analysis of results......................................... 146
Chapter seven: Conclusion and recommendations
7.1 Summary ....................................................................... 148
7.2 Achievements ........... 149
7.3 Recommendation .......................................................... 150
References .. 179
LISTS OF SYMBOLS AND ABBREVIATIONS
AENS Average energy not supplied per customer served
AI Artificial Intelligent
AM Asset Management
ANN Artificial Neural Network
ASAI Average Service Availability Index
CAIDI Customer Average Interruption Duration Index.
CAIFI Customer Average Interruption Frquency Index
CA Condition Assessment
CBM Condition Based Maintenance
CiGre International council on large electric systems
CM Corrective Maintenance
cm Condition Monitoring
CO Carbon monoxide
CO2 Carbon dioxide
DGA Dissolved gas analysis
DP Degree of polymerization
EPRI Electric Power Research Institute
FA Fura Analysis
FRA Frequency Response Analysis
HPP Homogeneous Poisson Process
HST Hot Spot temperature
HV High voltage
int. Interruption of voltage
LOE Average loss of energy
LTA Logic decision tree analysis
LV Low Voltage
MATLAB Matrix Laboratory
MC Monte Carlo
MM Maintenance Management
MTBF Mean Time Between Failures
MTTFF Mean Time To First Failure
MTTR Mean Time To Repair
MV Medium Voltage
NHPP Non Homogeneous Poisson process.
PHCN Power Holding Company of Nigeria
PD Partial Discharge
PM Preventive Maintenance
RCM Reliability-Centered Maintenance
RMS Root mean Square value
SAIDI System Average Interruption Duration Index.
SAIFI System Average Interruption Frequency Index
UMIST: University of Manchester Institute of Science and Technology.
LIST OF FIGURES AND DIAGRAMS
Figure 1.1: Project scope definition 12
Figure 2.1: Overview of maintenance approches 16
Figure 2.2: Reactive maintenance model 21
Figure 2.3: Proactive maintenance model 22
Figure 2.4: Composition of availability
and its controlling parameters .. 35
Figure 2.5: Logic of relating component maintenance
System reliability with operating costs. 44
Figure 3.1: Definitions of failures 50
Figure 3.2: Total time for repair/replacement 51
Figure 3.3: Outage time sequence 52
Figure 3.4: Discrete-parameter Markov Model for the determination of the
remaining life . 67
Figure 3.5: Continuous Parameter Markov Model 69
Figure 3.6: Function of the Mean Time to failure versus failure rate
Figure 3.7: Markov Model with Continuous Parameter 70
Figure 3.8: Diagram illustrating development of the mean transition time
between states i and j . 71
Figure 3.9: A simple maintenance model under deterioration failure. 73
Figure 4.1: Block diagram showing the origin of Ikeja
distribution zone . 83
Figure 4.2: Line diagram showing 10 injection substations .. 84
Figure 4.3: A section of the Abule-Egba distribution business unit 85
Figure 4.4: A typical customer feeder in Ojokoro Substation 86
Figure 4.5: Processed 2005 outage data for Abule-Egba business unit
Figure 4.6: Processed 2006 outage data for Abule-Egba business unit
Figure 4.7: Processed 2007 outage data for Abule-Egba business unit
Figure 4.8: Processed 2008 outage data for Abule-Egba business unit
Figure 4.9: Processed outage data for Ijaye Ojokoro feeders for 2005.92
Figure 4.10: Processed outage data for Ijaye Ojokoro feeders for 2006.93
Figure 4.11: Processed outage data for Ijaye Ojokoro feeders for 2007.
Figure 4.12: Processed outage data for Ijaye Ojokoro feeders for 2008
Figure 4.13a: Processed failure data for critical feeder for 2005....... 98
Figure 4.13b: Processed failure data for critical feeder for 2006 99
Figure 4.13c: Processed failure data for critical feeder for 2007 99
Figure 4.13d: Processed failure for critical feeder for 2008 100
Figure 5.1: Transformer maintenance model 109
Figure 5.2: Perfect Maintenance Model 113
Figure 5.3: Imperfect Maintenance Model 114
Figure 5.4: Inspection Model 114
Figure 5.5a-c: The relationship between inspection rate and MTTFF
Figure 5.6a-c: The relationship between inspection rate and MTTFF
When stage1 is represented by three subunits 119
Figure 6.1: Stages in the asset Management lifecycle. 123
Figure 6.2: Asset Management asset life cycle with about 90% Maintenance
Figure 6.3: Transformer asset Management activities 125
Figure 6.4: Transformer condition Monitoring and assessment techniques
Figure 6.5: Classification of Maintenance activities 129
Figure 6.6: Function of mean time to failure versus failure rate .. 132
Figure 6.7 Markov Model with Continuous Parameter 132
Figure 6.8: Markov Model for generating intensity Matrix 133
Figure 6.9: Estimated transformer life-span at varying failure rates. 142
Figure 6.10a-b: Sensitivity data fitted to 8th
degree polynomial and its
corresponding Norm residuals 142
Figure 6.11: Plot of the result of the sensitivity analysis when other variables are
held constant except maintenance rate () 145
Figure 6.12a-b: Sensitivity data fitted to 3rd
degree polynomial and its
corresponding Norm residuals 145
LIST OF TABLES
Table 2.1 Changing maintenance techniques .. 33
Table 4.1a: Statistical parameters from outage data set for 2004 96
Table 4.1b: Statistical parameters from outage dataset for 2005 96
Table 4.1c: Statistical parameters from outage dataset for 2006 96
Table4.1d: Statistical parameters from outage dataset for 2007 97
Table 4.1e: Statistical parameters from outage dataset for 2008 ..97
Table 5.1: Number of failures for each cause of failure 105
Table 5.2: List of the distribution of transformer failure by age .. 108
Table 5.3: Transformer maintenance tasks 110
Table 5.4: Rated limit for values of transformer oil for voltage class 111
Table 5.5: List of model parameters and definitions 112
The purpose of Maintenance is to extend equipment life time or at least the mean
time to the next failure.
Asset Maintenance, which is part of asset management, incurs expenditure but
could result in very costly consequences if not performed or performed too little.
It may not even be economical to perform it too frequently.
The decision therefore, to eliminate or minimize the risk of equipment failure
must not be based on trial and error as it was done in the past.
In this thesis, an enhanced Reliability-Centered Maintenance (RCM)
methodology that is based on a quantitative relationship between preventive
maintenance (PM) performed at system component level and the overall system
reliability was applied to identify the distribution components that are critical to
Maintenance model relating probability of failure to maintenance activity was
developed for maintainable distribution components. The Markov maintenance
Model developed was then used to predict the remaining life of transformer
insulation for a selected distribution system. This Model incorporates various
levels of insulation deterioration and minor maintenance state. If current state of
insulation ageing is assumed from diagnostic testing and inspection, the Model is
capable of computing the average time before insulation failure occurs.
The results obtained from both Model simulation and the computer program of
the mathematical formulation of the expected remaining life verified the
mathematical analysis of the developed model in this thesis.
The conclusion from this study shows that it is beneficial to base asset
management decisions on a model that is verified with processed, analysed and
tested outage data such as the model developed in this thesis.
1.1 Background Information
Ability to use electrical energy when required is one of the fundamental
presumptions of a modern society, and the introduction of complex and sensitive
machines and systems into the network had increased the need for high reliability
of supply . Deregulation and competition are forcing improvements in efficiency
and reductions in cost while customers are becoming more sensitive to electrical
disturbances and are demanding higher levels of service reliability. Since a typical
distribution system accounts for 40% of the cost to deliver power and 80% of
customer reliability problems, distribution system design, operations and
maintenance are critical for financial success and customer satisfaction .
Moreover, failure statistics  reveal that the electrical distribution systems
constitute the greatest risk to the uninterrupted supply of power. Traditionally
however, distribution systems have received less attention than the generation and
transmission parts of the overall electrical Power systems. This is emphasized by
the clear difference in the number of publications within the various relevant fields
The main reasons why distribution systems may not have been the centre of
focus are that they are less capital-intensive and that their failures cause more
localized effects compared to generation and transmission systems. However the
focus on generation and transmission systems is moving toward distribution as the
business focus changes from consumers to customers .
Electrical power systems have undergone major changes during the last few years
due to the introduction of the deregulated or liberalized market. (Sweden, for
example, was one of the first countries to deregulate its power- supply market.
This happened in January 1996) . This has implied that the driving factors have
moved from technical to economical. New players are now making their
appearance in the field. This fundamental and global-level change in the running
of power utilities has brought about diversity effects, including new opportunities
and new complications.
These utilities are themselves active in the deregulated market and face various
market challenges. For example, customers pay for energy delivered while
authorities impose sanctions/regulations, they supervise and they compensate
customers depending on the degree of fulfillment of contractual and other
obligations as recommended [6,7].
On the other hand, the owners expect the utilities to deliver at minimum
cost. This means that electricity utilities must satisfy quantitative reliability
requirements, while at the same time try to minimize their costs. One clear and
predominant expenditure for a utility is the cost of maintaining system assets, for
example through adopting preventive measures, collectively called preventive
maintenance (PM). Preventive maintenance measures can impact on reliability by
either, (a) improving the condition of an asset, or (b) prolonging the lifetime of an
asset . Reliability on the other hand, can be improved by either reducing the
frequency or the duration of power supply interruptions.
PM activities could impact on the frequency by preventing the actual cause of
the failure. Consequently, in cost- effective expenditure, PM should be applied
where the reliability benefits outweigh the cost of implementing the PM
Traditionally, preventive maintenance approaches usually consist of pre-defined
activities carried out at regular intervals (scheduled maintenance). Such a
maintenance policy may be quite inefficient; it may be costly (in the long run),
and it may not even extend component lifetime as much as possible. In the past
several years, therefore, many utilities replaced their maintenance routines based
on rigid schedules by more flexible program using periodic or even continuous
condition monitoring and data analysis . Research findings have shown that
maintenance impacts on the reliability performance of a component, that will
eventually reflect on the entire system since power systems is made up of
interconnected components. Many programs had been used to validate this
fact, such as failure effects analysis, an evaluation of needs and priorities, and
flow charts for decision making . Some of these approaches have been
collectively termed Reliability-Centred Maintenance (RCM) . In a RCM
approach, various alternative maintenance polices are compared and the most
cost-effective is selected.
RCM programs have been installed by many electric power utilities as a
useful management tool . However, the approach is still heuristic, and its
application requires judgment and experience at every turn. Also, it can take a
long time before enough data are collected for making such judgments. For this
reason, several mathematical models have been proposed to aid maintenance
Many of these models  deal with replacement policies only and disregard
the possibility of the cheaper but less effective maintenance activity. When
maintenance is modeled, most often, fixed maintenance intervals are assumed.
Only recently, was a mathematical model which incorporates the concept of
maintenance when needed was developed. Detailed literature reviews on the
various maintenance approaches and models are reported in references  and
In this research work, a probabilistic model was developed for the failure
and maintenance processes, and a Markov model for estimating the remaining
life of an identified critical component of distribution network was also
The model is based on a quantitative connection between reliability and
maintenance, a link missing in the heuristic approaches. This model is capable
of improving the decision process of a maintenance manager of network assets.
RCM strategies that are capable of showing the benefits of performing cost
effective PM on system networks on a selected system using reliability outage
data were performed so as to identify the cirtical component for analysis. The
model includes various levels of deterioration of the identified components as
well as maintenance and inspection states. Assuming that the present state of
component deterioration had been determined from diagnostic testing and
inspection, the model allows computation of the average time remaining before
failure occurs using a computer program developed in Matlab. Reliability-
centered Maintenance is a process used to determine the maintenance
requirements of any physical asset in its operating context. This is based on
equipment condition, equipment criticality and risk.
RCM provides a tool for maintenance management (MM) by using the model
to predict the remaining life of the identified component.
1.2 STATEMENT OF THE PROBLEM.
This project work addresses the importance of maintenance on the reliability of
electrical distribution systems. This focuses on preserving system function,
identifying critical failure modes, prioritizing important components and selecting
possible and effective maintenance activities, a cost effective preventive
maintenance plan which defines reliability centred maintenance.
1.2.1 Distribution Systems Constitutes The Greatest Risk.
Electric power system is not 100% reliable. The ability to use electric energy when
needed is the fundamental function of any modern utility company.
The existence of sophisticated machines and production lines had increased the
need for electricity supply that is highly reliable.
Distribution aspect of electricity system had been identified as constituting the
greatest risk to realizing uninterrupted power supply . Studies show that a
typical distribution system accounts for 40% of cost to deliver power and 80% of
customer reliability problems . This means that distribution systems are critical
for financial success and customer satisfaction. And yet distribution systems have
not received the desired attention. This was obvious from the difference in the
number of publications.
The main reasons advanced for the neglect of distribution systems include the
They are less capital intensive
Their failures cause more localized effects when compared with generation
and transmission systems and
1.2.2 Introduction of Liberalised Market.
The introduction of deregulated market has introduced major changes in electrical
power systems. These changes had led to the movement of the driving factors from
technical to economical. As a result, new investors are coming into the power
sector. This global level change in the running of power sector has brought about
new opportunities and new complications. In an increasingly competitive market
environment where companies emphasize cost control, operation and maintenance
(O&M), budgets are under constant pressure to economize. In order to ensure that
changing utility environment does not adversely affect the reliability of customer
power supply, several state regulatory authorities have started to specify minimum
reliability standards to be maintained by the distribution companies .
1.2.3 Cost-Effective Preventive Maintenance Expenditure
Electric power utilities own and operate system generation, transmission and
distribution of electricity. These utilities play active role in the deregulated market.
The implication of this is that they also face market requirements. This means that
customers will only pay for energy delivered. The Nigerian Electricity Regulatory
Commission (NERC) which is the monitoring authority in Nigeria will imposes
sanctions/regulations. Investors or Owners of utilities expect the managers to
deliver electricity to customers at minimum cost. This means that utilities must
satisfy reliability requirements at minimum cost. To achieve this, managers of these
utilities must consider maintenance cost for system assets as an important
expenditure area. Preventive maintenance measure is an activity undertaken
regularly at pre selected intervals while the device is satisfactorily operating to
reduce or eliminate the accumulated deterioration  while repair is the activity to
bring the device to a non failed state after it has experienced a failure. When the
cost incurred by a device failure is larger than the cost of preventive maintenance
(this cost could be cost of downtime, repair expenses, revenue lost etc.), then it is
worthwhile to carry out preventive maintenance.
Preventive Maintenance measures can affect reliability in two ways:
It helps in improving the condition of the asset and
It aids in prolonging the life time of an asset.
Effects of high reliability are:
It reduces the frequency of power outages by preventing the actual cause of
It also reduces the duration of power supply interruptions.
In cost effective expenditure, preventive maintenance applies where reliability
benefits outweigh the cost of implementing the preventive maintenance measures.
Traditional preventive maintenance is made up of pre defined activities carried
out at regular intervals. This type of maintenance is costly, inefficient and may not
even extend component lifetime. Many modern utilities have now replaced their
routine maintenance that is based on rigid schedules with a more flexible program
using periodic or even continuous condition monitoring (predictive maintenance).
The predictive maintenance routines include group of programs such as Failure
Modes and Effects Analysis, Evaluation of Needs and Priorities, and Flow Charts
for Decision Making are some approaches that have been named reliability- centred
maintenance (RCM) .
In RCM approach, different maintenance policies can be compared and the most
cost effective for sustaining equipment reliability selected. Reliability-Centred
Maintenance program is not new. This program has been installed by some electric
utilities as a useful management tool . The problem with those in existence is
that they cannot predict the effect of a given maintenance policy on reliability
indicators (failure rate, outage time etc) and the approach adopted is still heuristic.
This means that RCM in existence does not solve the fundamental problem of how
the system reliability is impacted by component maintenance.The application is
still based on experience and judgement at every turn. It takes a long time before
enough data are collected for making such judgements. To solve one of the
identified problems above, a probabilistic representation of deterioration process is
modeled. A new mathematical formulation of the expected transition time from any
deterioration state to the failure state (expected remaining life) has been presented.
Processed outage data obtained from a selected distribution network for a
component was used as input on the Markov model developed to predict the
remaining life. This predicted computation is executed using computer program
developed in Matlab.
Three stages will be used to describe deterioration process.
Stage 1 represents an initial stage (D1).
Stage 2 represents a minor deterioration stage (D2)
Stage 3 represents a major stage of deterioration (D3).
The last stage is followed in due time by equipment failure (F) which requires an
extensive repair or replacement. Maintenance is carried out on asset to slow down
deterioration. Inspections are performed so that decisions on asset management can
be taken. To run this model however, it was assumed that repair after failures
returns the device to the initial stage (as new condition). Figure 1.1 represents the
conceptual diagram of the probabilistic model.
Figure 1.1 conceptual diagram of the probabilistic model
1.3 AIM AND OBJECTIVES OF THIS STUDY
The aim of this research study is to develop an appropriate method that will aid
strategies for asset management in electric power distribution systems.
These methods/strategies when developed should be cost effective, balancing the
benefits in system reliability against the cost of maintenance methods. This will
lead to the utilization of the reliability-centred maintenance (RCM) method. This
method will be applied to specific parts in electrical power distribution systems.
The main objectives are to
a. Determine present maintenance policies in a selected distribution network.
b. Develop a probabilistic based model for maintenance strategies.
c. Predict the probable time of component failure, given that a certain stage in
the ageing process has already been reached.
d. Develop a quantitative relationship between preventive maintenance of
system components and overall system reliability.
e. Evaluate cost implications in the formulation of cost effective PM
f. Conduct program evaluation, including general application to electrical
power distribution systems.
1.4 RESEARCH METHODOLOGY
To fulfill the objectives of this work, the following methods will be adopted:
The first phase of the work is the system reliability analysis. This involves the
definition of the system and the identification of the critical components affecting
The second phase of the work is component reliability modeling. This entails
detailed analyses of the components with the support of appropriate input data
collected with the use of questionnaire. This will define the quantitative
relationship between reliability and preventive maintenance measures.
The third and final phase are system reliability and cost/benefit analyses: This is
carried out by putting the result of phase 2 into a system perspective, the effects of
component maintenance on system reliability will then be evaluated and the impact
on costs of different preventive maintenance strategies can now be identified.
1.5 SIGNIFICANCE OF THE STUDY
No doubt the country is in energy crisis, and the need to increase generation,
manage and upgrade the existing power infrastructure becomes imperative. The
costs of electric power outage to electric customers are enormous. Studies have
shown  that the cost of electricity failures on the Nigerian manufacturing sector
is quite high, as industries and firms incur huge costs on the provision of expensive
back up to minimize the expected outage cost. The average costs of this back up
are about three times the cost of publicly supplied power .
The main function of power utility is to supply customers with electrical energy at
high level of reliability at a reasonable cost. This intended function could be
affected by the problem of power outage, which is one of the measures of reliability
Power outage can; in principle be reduced in two ways:
By reducing the frequency of interruptions, that is the number of failures, or
By reducing the outage time, that is the duration of failure.
Application of RCM technique will be used to address the first aspect above, which
provides the focus for this study.
1.6 MOTIVATION FOR THE STUDY
Electricity is an aspect of the utility sector that is very essential to the smooth and
meaningful development of a society. It supports the economy and promotes the
well-being of individuals. Non-availability of this utility had led to a lot of
challenges ranging from lack of foreign investment, high cost of living since most
manufacturers depend on private generators, high rate of unemployment and
security and environmental hazards resulting from individuals generating their own
electricity without regulations.
A survey by Manufacturers Association of Nigeria (MAN) on power supply by the
power Holding Company of Nigeria (PHCN) to industrial sectors in the first
quarter of 2006 indicates that the average power outages increased from 13.3 hours
daily in January to 14.5 hours in March 2006 .
As at July 2009, Nigeria has total installed capacity of approximately 7060MW,
however, the country is only able to generate between 800MW 4000MW from
the seven major power stations and a numbers of IPP projects, because most of
these facilities have been poorly maintained. Nigeria has plans to increase
generation to 10,000MW by 2010. This means additional power plants, more
transmission lines, as well as more distribution facilities.
In recent times, subsequent governments of Nigeria had been working very hard to
see the realization of steady power supply in the country. For example, the
government of Chief Olusegun Obansanjo wanted to ensure an uninterrupted power
supply by the end of 2001 in Nigeria. The president then, made it clear when he
gave a mandate to the then National Electric Power Authority (NEPA) to ensure
uninterrupted power to the nation by 31st December 2001. It was noted that NEPA
actually raised electricity output from as low as 1,600 to 4,000MW and spent over
one million dollars to meet this mandate .
The present government also aware of this re-occurring power problem now
declared during his campaign days that he will declare a state of Emergency on
power sector when he assumed power. Yet as at February, 2010, Electricity
reliability and availability are still a mirage.
In this context, in as much as efforts are made towards efficient power generation,
the subsequent transmission and distribution of the generated power should not be
overlooked. Efficient utilization of the generated power cannot be achieved without
a sound maintenance plan and monitoring of the transmission and distribution
network system. Any organization that expects to run an efficient day to day
operation and to manage and develop its services effectively must know what
assets it has, where they are, their condition, how they are performing, and how
much it costs to provide the service . Knowledge about the physical assets of
the system is necessary to make strategic and maintenance/operation decisions.
Thus, to make an intelligent decision vital to the smooth operations, growth and
management of electricity distribution facilities, such decision must be based on a
model that is verifiable and quantifiable and should not be decisions based on
experience alone. This is the motivation for this project.
1.7 EXPECTED CONTRIBUTION TO KNOWLEDGE.
i. A selective maintenance method based on reliability analysis is being
ii. A Markov model for estimating the remaining life of a distribution
transformer is implemented using Matlab program.
iii. This will provide objectivity by converting the operators intuition into
quantifiable values that will aid in decision making process for asset
1.8 SCOPE AND LIMITATION.
This research work identified the following two ways in which the reliability of
electric supply to customers can be improved:
i. Reducing the frequency of power outages or
ii. Reducing the duration of power supply interruption.
This research work covers the first part that uses reliability centred maintenance
(RCM) to minimize the frequency of power outages by preventing the actual cause
of failure. This is shown in figure 1.2. Maintenance (one of the main tools of asset
management) in this context is considered as an activity of restoration where an
unfailed device has its deterioration arrested, reduced or eliminated
Its goal is to increase the duration of useful component life and postpone failures
that would require expensive repairs. For a successful operation of this RCM plan,
the degree of risk of each fault should be identified in order to define the optimum
maintenance actions. The type of maintenance action to be taken for a particular
asset will depend on the risk index of that asset. Critical component will be
identified from a selected network and the Markov model developed will be
applied on the identified component to predict the remaining life so as to make
intelligent decision on the asset.
Figure 1.2 Project scope definitions
1.9 THESIS ORGANIZATION
The overall thesis structure can be broken down into individual chapters as follows:
Chapter 1 provides an introduction, background studies, research methods and the
main contributions that are unique to this work.
Chapter 2 introduces and defines fundamental concepts for the analysis that
Chapter 3 introduces basic evaluation methods and techniques for reliability
modeling and analysis.
Chapter 4 presents the computer program developed for reliability analysis of the
electric power system.
Chapter 5 introduces different maintenance procedures/strategies and provides
introduction to reliability centred maintenance method (RCM) as applied to a
Chapter 6 presents results from comprehensive study of the causes of failures in the
identified critical components and then defines a model for estimating the
remaining life of the identified distribution transformer.
Chapter 7 concludes the work by summarizing the results obtained.
Recommendations and issues for future work are identified and discussed.
Asset management (AM) is a concept used today for the planning and operation of
the electrical power system. The aim of AM is to handle physical assets in an
optimal way in order to fulfill an organizations goal while at the same time
One of the major risks that are involved in asset management is the probability of
failure occurrence and its consequence. The goal is to ensure maximum asset value,
maximum benefit or minimal life cycle cost.
The only constraint to actualizing this goal is set on availability of revenues or
power supply. There are different possible actions of handling these assets: They
can either be acquired, maintained or replaced/redesigned.
Maintenance management (MM) is therefore defined as a strategy to handle
decisions for these assets and to make right decisions on
what assets to apply actions to.
what actions to apply
how to apply the actions
when to apply the actions
The purpose of maintenance is to extend equipment life time or at least the mean
time to the next failure whose repair may be costly. Further more, it is expected that
effective maintenance policies can reduce the frequency of service interruptions
and the many undesirable consequences of such interruptions. Maintenance clearly
affects component and system reliability: if too little is done, this may result in an
excessive number of costly failures and poor system performance and therefore
reliability is reduced: When done too often, reliability may improve, but the cost of
maintenance will sharply increase. In cost effective scheme, the two expenditures
must be balanced.
Maintenance is just one of the tools for ensuring satisfactory component and
system reliability. Others include increasing system capacity, reinforcing
redundancy and employing more reliable components. At a time, however, when
these approaches are heavily constrained, electric utilities are forced to get the most
out of the system they already own through more effective operating policies,
including improved maintenance programs. In fact, maintenance is becoming an
important part of what is often called asset management.
Electric utilities have always relied on maintenance programs to keep their
equipment in good working conditions for as long as it is feasible. In the past,
maintenance routines consisted mostly of pre-defined activities carried out at
regular intervals. (Scheduled maintenance). However such a maintenance policy
may be quite inefficient, it may be too costly (in the long run) and may not extend
component life time as much as possible. In the last ten years, many utilities
replaced their fixed interval maintenance schedules with more flexible programs
based on an analysis of needs and priorities, or on a study of information obtained
through periodic or continuous condition monitoring (predictive maintenance).
The predictive maintenance routines include a group of programs named
Reliability-Centred Maintenance, [RCM]. In an RCM approach, various alternative
maintenance policies are compared and the most cost-effective for sustaining
equipment reliability selected. RCM programs have been installed by several
electric utilities as a useful management tool.
The implementation of RCM programs represented a significant step in the
direction of getting the most out of the equipment installed. However, the
approach and procedure is still heuristic and its application requires experience and
judgment at every turn . Besides, it can take a long time before enough data are
collected for making such judgments. For this reason, several mathematical models
have been proposed to aid maintenance scheduling [22, 23, 24 and 30].
This chapter, gives a brief review of the most important approaches and models
described in the literatures. Next, present maintenance policies are then examined.
Subsequently, the use of mathematical models for maintenance strategies is
explored and desirable attributes of realistic probability-based models are listed. In
closing, definitions of the most important concepts discussed in the work are given.
2.2 MAINTENANCE APPROACHES.
A classification of the various maintenance approaches is presented in figure 2.1.
Maintenance is shown here as part of the overall asset management effort.
Maintenance policy is one of the operating policies and, in a given setting; it is
selected to satisfy both financial constraints.
ANALYSIS OF NEEDS
PURCHASING MAINTENANCE DISPOSAL
Figure 2.1 Overview of maintenance approachesMost of the discussion in the literature concerns replacements only, both after
failures and during maintenance, and they disregard the possibility of the kind of
maintenance where less improvement is achieved at smaller cost. The oldest
replacement schemes are the age replacement and bulk replacement policies .
In the first, a component is replaced at a certain age or when it fails, whichever
comes first. In the second, all devices in a given class are replaced at predetermined
intervals or when they fail. The last policy is easier to administer (especially if the
ages of components are not known) and may be more economical than a policy
based on individual replacement. Newer replacement schemes are often based on
probabilistic models   and can be quite complex. In most electrical utility
applications, however, maintenance resulting in limited improvement is an
established practice and replacement models have only a secondary role.
Maintenance programs range from the very simple to the quite sophisticated. The
simplest plan is to adopt a rigid maintenance schedule where pre-defined activities
are carried out at fixed time intervals. Whenever the component fails, it is repaired
or replaced. Both repair and replacement are assumed to be much more costly than
a single maintenance job. The maintenance intervals are selected on the basis of
long-time experience (not necessarily an inferior alternative to mathematical
models). To this day, this is the approach most frequently used.
The RCM approach referred to in the introduction is heavily based on regular
assessments of equipment condition and, therefore, does not apply rigid
maintenance schedules. The term RCM identifies the role of focusing maintenance
activities on reliability aspects. The RCM methodology provides a framework for
developing optimally scheduled maintenance programs. The aim of RCM is to
optimize the maintenance achievements (efforts, performance) in a systematic way.
This method requires maintenance plans and leads to a systematic maintenance
effort. Central to this approach is identifying the items that are significant for
system function. The aim is to achieve cost effectiveness by controlling the
maintenance performance, which implies a trade-off between corrective and
preventive maintenance and the use of optimal methods.
2.3 THE EMERGENCE OF RCM.
The RCM concept originated in the civil aircraft Industry in the 1960s with the
creation of Boeing 747 series of aircraft (the Jumbo). One prerequisite for obtaining
a license for this aircraft was having in place an approved plan for preventive
maintenance (pm). However, this aircraft type was much larger and more complex
than any previous aircraft type, thus PM was expected to be very expensive.
Therefore it was necessary to develop a new PM strategy. United Airlines led the
developments and a new strategy was created. This was primarily concerned
with identifying maintenance tasks that would eliminate the cost of
unnecessary maintenance without decreasing safety or operating performance.
The resulting method included an understanding of the time aspects in reliability
(ageing) and identifying critical maintenance actions for system functions. The
maintenance program was a success. The good outcome raised interest and the
program spread. It was further improved, and in 1975 the US Department of
commerce defined the concept as RCM and declared that all major military systems
should apply RCM. The first full description was published in 1978 , and in the
1980s the Electric Power Research Institute (EPRI) introduced RCM to the Nuclear
power industry. Today RCM is under consideration by, or has already been
implemented by many electrical power utilities for managing maintenance
2.4 EVOLUTION OF MAINTENANCE
RCM provides a framework which enables users to respond to maintenance
challenges quickly and simply. It does so because it never loses sight of the fact
that maintenance is about physical assets. If these assets did not exist, the
maintenance function itself would not exist. So RCM starts with a comprehensive,
zero-based review of maintenance requirements of each asset in its operating
All too often, these requirements are taken for granted. This results in the
development of organization structures, the deployment of resources and the
implementation of systems on the basis of incomplete or incorrect assumptions
about the real needs of the assets. On the other hand, if these requirements are
defined correctly in the light of modern thinking, it is possible to achieve quite
remarkable step changes in maintenance effectiveness.
The meaning of maintenance is explained. It goes on to define RCM and to
describe the seven key steps involved in applying this process.
Maintenance and RCM
Considering the engineering view points, there are two elements to the
management of any physical asset. It must be maintained and from time to time it
may also need to be modified.
2.4.1 What Is Maintenance?
Every one knows what maintenance is, or at least has his own customized
definition of maintenance. If the question is asked, words like fix, restore, replace,
recondition, patch, rebuild, and rejuvenate will be repeated. And to some extent,
there is a place for these words or functions in defining maintenance. However, to
key the definition of maintenance to these words or functions is to miss the mark in
understanding maintenance especially if you wish to explore the philosophical
nature of the subject.
Maintenance is the act of maintaining. The basis for maintaining is, to keep,
preserves, and protect. That is, to keep in an existing state or preserve from failure
or decline. There is a lot of difference between the thoughts contained in this
definition and the words and functions normally recalled by most people who are
knowledgeable of the maintenance function, ie fix, restore, replace, recondition,
Maintenance can therefore be defined, as ensuring that physical assets continue to
do what their users want them to do.
What the users want will depend on exactly where and how the asset is being used
(the operating context). Maintenance procedures are an integrated part of the
planning, construction and operation of a system. Moreover they are central and
crucial to the effective use of available equipment. The aim of maintenance
activities is to continuously meet performance, reliability and economic
requirements, while also adhering to the constraints set by system and customer
The maintenance concept refers to all actions undertaken to keep or restore
equipment to a desired state. The electrical power systems must abide by the
regulations and norms for heavy current and maintenance, and in Nigeria, must
follow the IEE regulations standard code. The IEE standard is the regulation
governing the planning, building and maintenance of power distribution systems
for 0.415 33kV. The first IEE standard regulation was created in the 1960s and
the new handbooks have recently been developed to support more effective
The cost of maintenance must be taken into consideration when handling system
assets to minimize the lifetime costs of the system. However, some maintenance
activities must be undertaken even if they are not profitable, such as earth plate
metering inspections stipulated in the IEE regulations for power system. .
There are two types of maintenance: Preventive Maintenance and Corrective
Preventive Maintenance can be planned and scheduled, but corrective maintenance
occurs unpredictably when failures are detected. This thesis focuses on preventive
2.4.2 Maintenance Approaches
From a basic point of view, there are two maintenance approaches. One approach is
reactive and the other is proactive. In practice, there are many combinations of the
basic approaches. The reactive system whose model is shown in figure 2.2
responds to the following:
a work request order
Production staff identified needs.
Failed system or its component.
The effectiveness of this system will depend on response measures. The goals of
this approach are to reduce response time to a minimum and to reduce equipment
down time to an acceptable level.
This is the approach used by most operations today. It may well incorporate what is
termed as a preventive maintenance program and may use proactive technologies.
THE BASICS OF MAINTENANCE AND RELIABILITY
NOTIFICATION PLANNING SCHEDULING MECHANIC
FIGURE 2.2 REACTIVE MAINTENANCE MODEL
The proactive approach (figure 2.3) responds primarily to equipment assessment
and predictive procedures. The overwhelming majority of corrective,
preventative and modification work is generated internally in the maintenance
function as a result of inspections and predictive procedures.
The goals of this method are continuous equipment performance to established
specifications, maintenance of productive capacity, and continuous improvement.
Figure 2.3 Proactive Maintenance Model
PLANNED SCHEDULED PREVENTIVE MAINTENANCE
2.4.3 Changing Maintenance Trends.
The International Council on Large Electric Systems (CIGRE) is one of the leading
worldwide Organizations on Electric Power Systems with headquarters in France.
This is a permanent non-governmental and non-profit international association that
was founded in 1921. One of CIGRE core mission issues is related to the planning
and operation of power systems, as well as design, construction and maintenance of
the plants. Technical work is being carried out within 15 study committees.
One of these working groups set up a questionnaire in 1997 to obtain more
information about trends in future power system planning, design, operation,
maintenance, extension and refurbishment.
A summary of this report can be found in reference , based on about 50
responses obtained from utilities, manufacturers and consultants. Some of the
results of particular importance to this context are pointed out in the following
It is evident in the results that utilities have changed their organization in response
to deregulation. The primary changes of note include the privatization of companies
and splitting up of generation and distribution activities. The intense pressure to
reduce operational and maintenance costs has already been felt. Maintenance,
design, construction and some aspect of operation are increasingly being contracted
out. The driving forces behind these changes are more aligned institutional business
and economic factors than technical considerations.
Another projected trend identified in the results is that manufacturers will become
increasingly incorporated into the maintenance systems.
Some of the figures presented in the report are as shown below:
Almost 40% of the utilities undertake their maintenance activities at fixed
time intervals and 30% on monitoring conditions. Many utilities falling into
the first category are evolving towards condition or system- reliability based
maintenance, or both.
About half the utilities and all the manufacturers that responded have
performed reliability studies to optimize their maintenance. These reliability
studies resulted in introducing more flexibility and diversity into the
In the past, utilities have laboured to achieve maximum reliability.
However, according to the responses, about 90% thought that aiming for
optimal and thereby more specific reliability in different parts of the
system is the trend for the future.
Data concerning the times for repair and maintenance were stated to be
available, but data on failure modes were claimed to be more difficult to
This study provides a similar picture of the maintenance issue to that identified in
the introduction of this thesis. It reveals a changing situation with increasingly
complicated systems that are driven by economic rather than technical factors, and
with the overall objective of achieving cost effective expenditures rather than
2.4.4 Changing Requirement for Maintenance Methods
The change in the way maintenance is being managed has been identified. This
change implies greater requirements on maintenance procedures. For example,
maintenance decisions have been traditionally based on experiences and
measurements which could be supported by diagnostic method. The increase in the
expectations of maintenance has kept pace with the increasing knowledge about the
dynamic characteristics of the power system. These higher expectations are due to
the increasingly complex systems and higher demands on cost-effective use of
resources. The increasing knowledge about the system has been gained primarily
by an understanding of the relationships between failure frequency, reliability and
maintenance, and also by methods and continuous measurements.
2.4.5 Maintenance Specifications and Performance
To explain maintenance specifications, maintenance definition will be considered
in the context of keeping, preserving and protecting machine, equipment or plant.
The challenge often faced in an attempt to perform these tasks is how to define the
level to which the machine, equipment or plant is to be kept. One of the most
common ways would be to say keep it like new. This sounds good, but it is more
subjective than objective. To answer this issue of maintenance level, leads to
Specification is a detailed precise presentation of that which is required. We must
have a specification for the maintenance of equipment and plant. Specifications
usually exist in the mind of the maintenance Engineer, even though they may be
unable to recite it. This type of specification is defined in terms of and is dependent
upon time available, personnel training level, pressure to produce a current
customer order now, money allocated or available, or management opinion.
Obviously, a specification like this will not qualify as a true specification, nor will
it qualify as a supporting component of the act of maintaining. The true
maintenance specification may be a vendor specification, a design specification or
an internally developed specification. The specification must be precise and
objective in its requirement.
The maintenance system and organization must be designed to support a concept
based on acceptable standard. Specifications, detailed work plans and schedules
may be constructed to provide the specification requirement at the maintenance
level. In the maintaining context, specification is not a goal. It is a requirement that
must be met. The maintenance system must be designed to meet this requirement.
The specification must be accepted as the floor or minimum acceptable
maintenance level. Variation that does occur should be above the specification
level or floor. The specifications will probably be stated in terms of attributes and
In reference to maintenance specifications, individual equipment specifications,
process specification and plant performance specifications are also included.
2.4.6 The Maintenance Function
The maintenance department is responsible and accountable for maintenance. It is
responsible for the way equipment runs and looks and for the costs to achieve the
required level of performance. This is not to say that the operator has no
responsibility for the use of equipment under his custody. The point is that
responsibility and accountability must be assigned to a single function or person
whether it is a mechanic or operator. To split responsibility between maintenance
or any other department where overlapping responsibility occurs is to establish an
operation where no one is accountable.
The maintenance function is responsible for the frequency and level of
maintenance. They are responsible for the costs to maintain, which requires
development of detailed budgets and control of costs to these budgets.
Where the maintenance department or group is held responsible and accountable
for maintenance, the relationship with other departments takes on new meaning.
They must have credibility and trust as basis of interdepartmental relationships.
This is an essential element for the successful operation of a maintenance
2.5 WHAT IS RELIABILITY?
Most maintenance professionals are intimidated by the word reliability, because
they associate reliability with RCM (Reliability-Centred Maintenance) and are
unclear on what it actually means.
Reliability is the ability of an item to perform a required function under a stated set
of conditions, for a stated period of time . However, many utilities focus on
fixing equipment when it has already failed rather than ensuring reliability and
A common reason for this finding is the lack of time to investigate what is needed
to ensure the reliability of equipment. Yet, a growing awareness among these
reactive maintenance organizations is that the consequences of poor equipment
performance include higher maintenance costs, increased equipment failure, asset
availability problems and safety and environmental impacts. There is no simple
solution to the complex problem of poor equipment performance. The traditional
lean manufacturing or world class manufacturing is not the answer. These
strategies do not address the true target, but if we focus on asset reliability, the
result will follow.
2.5.1 Reliability-Focus Utilities
It is not possible to manage today power system operation with yesterday methods
and remain in business tomorrow. Most chief executive of Companies that are
doing well decide to focus on reliability because maintenance is the largest
controllable cost in an organization  and, without sound asset reliability, losses
multiply in many areas. A research carried out by over 50 key employees of the
worlds best maintenance organizations for a period of two years revealed the
When the best practices they found were assimilated and implemented in a
disciplined and structured environment, it was found to offer the biggest return with
the longest lasting results.
Corporations that truly understand reliability typically have the best performing
plants. Some of the characteristics of reliability-focused organizations are
Their goal is optimal asset health at an optimal cost.
They focus on processes what people are doing to achieve results.
They measure the effectiveness of each step in the process, in addition to
Their preventive maintenance programs focus mainly on monitoring and
managing asset health.
Their preventive maintenance programs are technically sound with each
task linked to a specific failure mode, formal practices and tools are used to
identify the work required to ensure reliability.
2.5.2 System Functional Failure and Criticality Ranking.
The objective of this task is to identify system functional degradation and failures
and rank them as to priority. The functional degradation or failure of a system for
each function should be identified, ranked by criticality and documented.
Since each system functional failure may have a different impact on availability,
safety and maintenance cost, it is necessary to rank and assign priorities to them.
The ranking takes into account probability of occurrence and consequences of
failure. Qualitative methods based on collective Engineering judgment and the
analysis of operating experience can be used. Quantitative methods of simplified
failure modes and effects analysis (SFMEA) or risk analysis also can be used.
The ranking represents one of the most important tasks in RCM analysis. Too
conservative ranking may lead to an excessive preventive maintenance program,
and conversely, a lower ranking may result in excessive failures and potential
safety impact. In both cases, a nonoptimized maintenance program results.
2.6 RELIABILITY-CENTERED MAINTENANCE
Reliability-centered Maintenance is a process used to determine the maintenance
requirements of any physical asset in its operating context.
2.6.1 RCM Method
RCM provides a formal framework for handling the complexity of the maintenance
issues but does not add anything new in a strictly technical sense. RCM principles
and procedures can be expressed in different ways ; however, the concept and
fundamental principles of RCM remain the same.
The RCM method facilitates the
preservation of system function,
identification of failure modes,
prioritizing of function needs, and
selection of applicable and effective maintenance tasks.
Several different formulations of the process of creating an RCM program and
achieving an optimally-scheduled maintenance program were found in the
literature. Three of these formulations had been addressed. The first two were
derived from the original RCM definitions, and the third is an approach based
on a set of questions rather than steps.
Smith defined a systematic process for RCM by implementing the following
features that have been defined above:
1. System selection and information collection.
2. System boundary definition.
3. System description and functional block diagrams.
4. System functions and functional failures.
5. Failure mode and effects analysis (FMEA).
6. Logic decision tree analysis (LTA).
7. Selection of maintenance tasks.
Nowlan defines the process of developing an initial RCM program when the
information required is lacking, as follows :
(1) Partitioning the equipment into object categories in order to identify those items
that require intensive study,
(2) Identifying significant items that have essential safety or economic
consequences and hidden functions that require scheduled maintenance.
(3) Evaluating the maintenance requirements for each significant item and
hidden function in terms of the failure consequences and selecting only
those tasks that will satisfy these requirements.
(4) Identifying items for which no applicable or effective task can be found,
then either recommending design changes if safety is involved, or assigning
no scheduled maintenance tasks to these items until further information
(5) selecting conservative initial intervals for each of the included tasks
grouping the tasks in maintenance packages for application,
(6) Establishing an age-exploration program to provide the factual information
necessary to revise initial decisions.
The first step is primarily an activity for reducing the problem to a manageable
size. The following three steps stated above are the essence of RCM analysis,
constituting the decision questions as stated by Moubray in (3) below.
To analyse the maintenance aspects of a system and its components, the first step is
to identify the system items, and which of these ought to be analysed. Thereafter
the RCM process can be formulated into seven questions for each of the selected
The seven general questions are:
1. What are the functions and performances required?
2. In what ways can each function fail?
3. What causes each functional failure?
4. What are the effects of each failure?
5. What are the consequences of each failure?
6. How can each failure be prevented?
7. How does one proceed if no preventive activity is possible?
2.6.2 Failure Consequences
A detailed analysis of an average industrial undertaking is likely to yield between
three and ten thousand possible failure modes. Each of these failures affects the
organization in some way, but in each case, the effects are different. They may
affect operations. They may also affect product quality, customer service, safety or
the environment. They will all take time and cost money to repair.
It is these consequences which most strongly influence the extent to which we try
to prevent each failure. In other words, if a failure has serious consequences, we are
likely to go to great lengths to try to avoid it. On the other hand, if it has little or no
effect, then we may decide to do no routine maintenance beyond basic cleaning and
A great strength of RCM is that it recognizes that the consequences of failures are
far more important than their technical characteristics. In fact, it recognizes that
the only reason for doing any kind of proactive maintenance is not to avoid
failures per se, but to avoid or at least to reduce the consequences of failure.
The RCM process classifies these consequences into four groups, as follows:
Hidden failure consequences: This has no direct impact, but they expose the
organization to multiple failures with serious, often catastrophic,
consequences. (Most of these failures are associated with protective devices
which are not fail-safe.)
Safety and environmental consequences: A failure has safety consequences
if it could hurt or kill someone. It has environmental consequences if it
could lead to a breach of any corporate, regional, national or international
Operational consequences: A failure has operational consequences if it
affects production (output, product quality, customer service or operating
costs in addition to the direct cost of repair).
Non-operational consequences: Evident failures which fall into this
category affect neither safety non production, so they involve only the
direct cost of repair.
2.6.3 Growing Expectation of Reliability-Centered Maintenance
Since the 1930s, the expectation of maintenance can be traced through three
generations. RCM is rapidly becoming a cornerstone of the third Generation, but
this generation can only be viewed in perspective in the light of the first and second
The first Generation
The first generation covers the period up to World War II. In those days industry
was not highly mechanized, so downtime did not matter much. This meant that the
prevention of equipment failure was not a very high priority in the minds of most
managers. At the same time, most equipment was simple and much of it was over-
designed. This made it reliable and easy to repair. As a result, there was no need for
systematic maintenance of any sort beyond simple cleaning, servicing and
lubrication routines. The need for skills was also lower than it is today.
The Second Generation
Things changed dramatically during World War II. Wartime pressures increased
the demand for goods of all kinds while the supply of industrial manpower dropped
sharply. This led to increased mechanization. By the 1950s, machines of all types
were more numerous and more complex. Industry was beginning to depend on
As this dependence grew, downtime came into sharper focus. This led to the idea
that equipment failures could and should be prevented, which led in turn to the
concept of preventive maintenance. In the 1960s, this consisted mainly of
equipment overhauls done at fixed intervals.
The cost of maintenance also started to rise sharply relative to other operating
costs. This led to the growth of maintenance planning and control systems. These
have helped greatly to bring maintenance under control, and are now an established
part of the practice of maintenance.
Finally, the amount of capital tied up in fixed assets together with a sharp increase
in the cost of that capital led people to start seeking ways in which they could
maximize the life of the assets.
The Third Generation
Since the mid-seventies, the process of change in industry has gathered even
greater momentum. The changes can be classified under the headings of new
expectations, new research and new techniques.
Downtime has always affected the productive capability of physical assets by
reducing output, increasing operating costs and interfering with customer service.
By the 1960s and 1970s, this was already a major concern in the mining,
manufacturing and transport sectors. In manufacturing, the effects of downtime are
being aggravated by the worldwide move toward just-in-time systems, where
reduced stocks of work-in-progress mean that quite small breakdowns are now
much more likely to stop a whole plant. In recent times, the growth of
mechanization and automation has meant that reliability and availability have now
also become key issues in sectors as diverse as health care, data processing,
telecommunications, power systems and building management.
Greater automation also means that more and more failures affect our ability to
sustain satisfactory quality standards. This applies as much to standards of service
as it does to product quality.
More and more failures have serious safety or environmental consequences, at a
time when standards in these areas are rising rapidly. In some parts of the world,
the point is approaching where organizations either conform to societys safety and
environmental expectations, or they cease to operate. This adds an order of
magnitude to our dependence on the integrity of our physical assets one which
goes beyond cost and which becomes a simple matter of organizational survival.
At the same time as our dependence on physical assets is growing, so too is their
cost to operate and to own. To secure the maximum return on the investment
which they represent, they must be kept working efficiently for as long as we want
Finally, the cost of maintenance itself is still rising, in absolute terms and as a
proportion of total expenditure. In some industries, it is now the second highest or
even the highest element of operating costs . As a result, in only thirty years it
has moved from almost nowhere to the top of the league as a cost control priority.
Table 2.1 Changing maintenance techniques
Fix it when
Systems for planning
Big, slow computers
Design for reliability and
Small, fast computers
Failure modes and effects analyses
Multiskilling and teamwork
Apart from greater expectations, new research is changing many of our most basic
beliefs about age and failure. In particular, it is apparent that there is less and less
connection between the operating age of most assets and how likely they are to fail.
However, Third Generation research has revealed that not one or two but six failure
patterns actually occur in practice.
There has been explosive growth in new maintenance concepts and techniques.
Hundreds have been developed over the past fifteen years, and more are emerging
2.7 RELIABILITY ENGINEERING
The general power evaluation term reliability, such as availability, can be seen as
a combination of three factors: Reliability of a piece of equipment or a part of the
system, maintainability, which is the possibility to detect failures and to read and
restore the components and the maintenance support or supportability i.e. spare
parts, maintenance equipment and the ability of the maintenance staff. The
availability concept and parameters of importance are illustrated in figure 2.4. All
three areas are affected when underground cables replace overhead lines.
80 % of the failures in distribution network are related to the electrical components,
such as overhead lines, cable systems, secondary substations or medium voltage
switchgear stations,  these components are made up of different parts of which
all have a probability to fail. Cable system faults are not only faults on the cables
but also on joints and terminations. In addition to the condition of individual
components, network topology and environmental factors influence the ability of
the system to perform a required function.
FIGURE 2.4 Composition of availability and its controlling parameters.
2.8 THE RELIABILITY ENGINEERING PROCESS.
One approach to reliability engineering is to divide the process into four basic
Past system behaviour
Reliability calculation methods.
Calculation of reliability indices and
Prognosis of future system. 
It is mainly the activities in step one, the collecting of data in order to create models
of outages and failures, that differ between networks with an extensive amount of
cable and traditional overhead line networks. The failure rates of different
components, calculated in step one, are of the engineering process.
2.9 RELIABILITY EVALUATION
In most of the literature, the fundamental problem area considered is that of failure
events in electric power systems. To make the analysis of this fundamental issue
possible, abstract models were created using mathematical language instead of
presenting the problem analogously. An abstract model can either be deterministic
or probabilistic. In a deterministic model, reality can be approximated with a
mathematical function. In a stochastic or random model, the unknown behaviour is
included in the model. Probability theory is used to analysize